1. Field of the Invention
The present invention relates generally to transmission of broadcast television signals using non-linear devices as high power amplifiers, as well as RF signal combiners for combining two or more signals for propagation through a common propagation device, such as a broadcast tower antenna, and more particularly to methods and systems for precorrecting input signals before they are inputted to a combiner for transmission to an antenna for propagation.
2. Background and Related Art
It is generally known in the art to compensate for distortions introduced to input signals (such as composite television signals) by non-linear devices used to amplify such signals, by precorrecting such signals prior to inputting them to the non-linear device. See, e.g., U.S. Pat. No. 5,198,904.
The benefits of inductive output tubes (IOTs) as power amplifiers in television transmitters, especially in common amplification configurations, are well known in terms of increased efficiency and reliability. However, a characteristic of the IOT which must be compensated for is its nonlinearity.
A correction scheme known as Digital Adaptive Correction (DAP) has been proposed to compensate for signal distortions caused by such non-linear amplifying devices. Pursuant to DAP, the output signal of a non-linear amplifier is measured and compared with an ideal signal at the output of a modulator. The difference between the ideal signal and the measured output signal represents error caused by the non-linear amplifier. This difference could be stored, such as in a Look-Up Table (LUT), against magnitude and/or phase values of the input signal. Thereafter, signals being inputted to the modulator would be measured, the appropriate correction value would be retrieved from the LUT, and applied to the input signal before being amplified by the non-linear device.
Many applications exist where it is desired to combine signals of various frequencies into a common transmission line structure. One example of such an application is in the field of microwave communications. One requirement of such combination is that the different signals must not improperly interfere with each other.
In this regard, various tuned filtering devices have been used in the prior art. However, due to the non-ideal nature of some of such devices, unwanted distortions and losses are inevitably introduced into the frequency bands of the different signals, causing the signals to be degraded in quality.
One particular application for combining electromagnetic signal waves has arisen with the advent of digital television (DTV). Broadcasters who wish to provide DTV service still must provide broadcast signals in the conventional NTSC (National Television Systems Committee) format to accommodate viewers with conventional NTSC television receivers. Absent a feasible and efficient way to combine the DTV signal and the NTSC signal for transmission by a common broadcast antenna (possibly on adjacent frequency channels) without causing unacceptable interference, interaction and distortion of the signals, this will require some broadcasters to build additional antennae and towers to carry their DTV signals. This represents a very expensive and undesirable proposition.
Under present NTSC broadcast allocations, each active channel of 6 MHZ bandwidth is separated from other channels by one or more 6 MHZ wide inactive channels, in order to prevent interchannel interference between signals. Because of the limited nature of the electromagnetic spectrum and the requirement for broadcasters to continue to provide conventional NTSC signals to serve viewers who do not own DTV receivers, DTV signals will, in some cases, be broadcast on currently inactive 6 MHZ wide channels adjacent to the lower and upper sides of current NTSC broadcasting channels.
Recently, highly selective RF signal combiners have been proposed for use in adjacent channel applications. Similar to non-linear devices such as IOTs, however, such signal combiners also can degrade signal performance through the process of signal combination. Use of adaptive precorrection on signals being inputted to RF signal combiners would require that a signal sample be taken at the output of the combiner to be used in the adaptive precorrection circuit. However, because the output of the combiner is by its nature a combined signal consisting of two or more separate broadcast signals, the sample would contain the additional signals along with the desired signal to be precorrected. The presence of such undesired additional signals in the sample will at best significantly degrade the adaptive precorrection, and at worst would completely debilitate the function of the adaptive precorrection circuit. Attempted removal of the undesired signals by filtering is difficult or impossible without distorting the desired signal sample itself.
Consequently, there exists a need in the art to provide precorrection of signals being inputted to RF signal combiners to counteract the degradation of the signals by the signal combiner.
The present invention provides a solution to the shortcomings of the prior art as discussed above.
In particular, the present invention provides a circuit for precorrecting a first signal in a broadcast signal transmitter for transmitting a combined signal on a common propagation device, including an amplifying device for amplifying a first signal to provide an amplified first signal, an RF combiner for combining the amplified first signal with at least a second signal to provide at an output of the combiner a combined signal for propagation on a propagation device, an adaptive precorrector for receiving the first signal at a first input thereof and precorrecting the first signal for distortions introduced into the first signal by the amplifying device and the RF combiner in accordance with a signal sample received at a second input thereof, an output of the precorrector being applied to the amplifying device, and a hybrid which receives at one input thereof a sample of the combined signal, and which receives at a second input thereof a sample of the second signal in a manner so as to provide at an output thereof a sample of the first signal constituting the signal sample provided to the adaptive precorrector.
According to another aspect of the invention, a method is provided for precorrecting a first signal in a broadcast signal transmitter for transmitting a combined signal on a common propagation device, comprising the steps of combining a precorrected first signal with at least one additional signal to provide a combined signal for propagation on a propagation device, applying to one input of a hybrid a sample of the combined signal, applying at a second input of the hybrid a sample of the at least one additional signal in a manner so as to provide at an output of the hybrid a sample of the first signal, and applying the sample of the first signal to a precorrector which receives the first signal and precorrects the first signal, in accordance with the sample, for distortions introduced into the first signal by the RF combiner, and produces at an output thereof the precorrected first signal which is applied to the RF combiner.